The Climate Irreversibility and Regional Characteristics in the Next Fifty Years

Significant gaps remain in our understanding of the mechanisms driving temperature reversibility and existence of climate tipping point. This study aims to quantify and understand the global and regional Earth’s climate reversibility using a fully-coupled climate model (NCAR CESM2). Starting in 2015, we increase CO2 concentrations following the SSP585 scenario until 2075, while other anthropogenic emissions are fixed at 2015 level. For each 10-year interval between 2025 and 2075, we branch the experiment and reverse the CO2 trend (i.e. decrease CO2 at the same rate as it increases) until it returns to 2015 level. The model reveals that the 60°S to 45°S latitude band, corresponding to the Southern Ocean (SO) region, exhibits the lowest climate reversibility in terms of surface temperature. It shows that the cloud fraction decreases when CO2 increases while low recovery when CO2 trend is reversed, which is the main cause of the low reversibility of radiative imbalances at the surface and thus surface temperature. Further analyses indicate that the microphysical properties of ice in the mixed-phase cloud, such as ice water content and effective radius, are highly irreversible. It infers that the efficiency of Wegener–Bergeron–Findeisen (WBF) process, which governs the growth of ice particles in mixed-phase SO clouds, may play an important role in SO clouds and regional climate reversibility.